Carbon nanotubes are seamless tubes of graphite sheets and can be either multi-walled (MWNT) or single-walled (SWNT). Generally, carbon SWNT are preferred over carbon MWNT's because SWNT have fewer defects and are stronger and are better electrical conductors than MWNT's of similar diameter. The structure of the SWNT is defined by how the graphite sheet is aligned in a rolled up configuration. Carbon nanotubes exhibit technologically important electronic properties and have shown promising applications, including nanoscale electronic devices, high strength materials, electron field emission devices, tips for scanning probe microscopy, and chemical sensors. Most electronic applications of carbon nanotubes require significant amounts of aligned SWNT that are reasonably homogeneous in diameter, length and helicity, since the electronic properties correlate both with the diameter and chirality (twist). Carbon nanotubes can be found in both metallic and semiconducting structures. Metallic nanotubes can carry large current densities while semiconducting nanotubes can be electrically switched on and off like field-effect transistors (FET's).
Various approaches for synthesizing carbon nanotubes are described, for example, in U.S. Pat. No. 6,333,016 which is incorporated herein by reference.
Single-walled carbon nanotubes are typically prepared in the presence of a particulate transition metal catalyst, such as V or Co. However, SWNT prepared using the particulate catalysts show a rather broad distribution of SWNT diameters, with the width of the distribution increasing with the SWNT diameter. Catalysts, in particular metal catalysts, tend to restructure and sinter under the harsh reaction conditions required for SWNT synthesis, leading to the formation of multi-faceted crystals, with each facet potentially initiating the growth of a SWNT and contributing to the heterogeneity in diameter and structure.
As a result, no economically scalable methods exist for reliably preparing, separating or aligning nanotubes of controlled diameter and electronic properties either by selective synthesis or through post-synthesis separation. The inability to make controlled junctions on the nanoscale is also a limiting factor in making particular electronic devices requiring local gate layout. Device development therefore is limited by the lack of control in synthesizing clean aligned nanotubes of a specified type. The inability to make controlled junctions on a nanometer scale is also a limiting factor in making particular electronic devices requiring local gate layout.
It would therefore be desirable to develop a material system and a process for the growth of carbon SWNT with better defined chemical and physical properties. It would also be desirable to produce nanoscale devices that incorporate such SWNT for electronic and sensing applications.